There has heretofore been known a vehicular drive system, having two shifting mechanisms including a continuously variable transmission and a step-variable shifting portion, through which an output of a drive power source is transmitted to drive wheels. With such a vehicular drive system, in general, an overall speed ratio of the drive system is established based on speed ratios of the shifting mechanisms.
As one example of the continuously variable transmission, there has heretofore been known a drive system, including a differential mechanism for distributing an output of an engine to a first electric motor and an output shaft, and a second electric motor disposed between an output shaft of the differential mechanism and drive wheels, which functions as a continuously variable transmission. For instance, Patent Publication 1 (Japanese patent Application Publication No. 2003-301731) discloses a hybrid vehicular drive system that corresponds to such a drive system mentioned above.
With such a hybrid vehicular drive system, a differential mechanism is comprised of, for instance, a planetary gear set, which performs a differential action to mechanically transmit a major part of drive power from the engine to the drive wheels while a remnant of drive power from the engine is electrically transmitted from the first electric motor to the second electric motor using an electric path. This causes a control device to allow the differential mechanism to function as an electrically controlled continuously variable transmission to electrically and continuously vary the speed ratio. This allows the engine to operate in an optimum operating state while enabling a vehicle to run, thereby enabling improvement in fuel consumption.
One example of the vehicular drive system having two shifting mechanisms, there has heretofore been known a drive system including, for instance, a step-variable automatic transmission (hereinafter referred to as “a step-variable transmission”) as a transmission incorporated in a power transmitting path between an output member of an electrically controlled continuously variable transmission and drive wheels. For instance, Patent Publication 2 (Japanese patent Application Publication No. 2003-130203) discloses a hybrid vehicular drive system that corresponds to such a drive system mentioned above.
With such a hybrid vehicular drive system, the drive system has an overall speed ratio determined based on a speed ratio of the electrically controlled continuously variable transmission and a speed ratio of the step-variable transmission. With a shifting of the electrically controlled continuously variable transmission independently controlled like a drive system equipped only with an electrically controlled continuously variable transmissions whole of the drive system is caused to function as the electrically controlled continuously variable transmission so as to allowing the vehicle to nm with the engine controlled in an optimal operating state.
During the shifting of the electrically controlled continuously variable transmission or during the operation of the step-variable transmission independently effectuated to perform the shifting, the engine rotation speed was caused to vary step-by-step with a stepwise variation in speed ratio. This resulted in a likelihood that it became difficult for the drive system as a whole to ensure continuity of the speed ratio on a stage before and after the shifting of the step-variable transmission. In other words, it was likely that the drive system could not function as a whole to operate as the continuously variable transmission on the stage before and after the shifting of the step-variable transmission. Thus, there was a likelihood that it became difficult to control the engine rotation speed so as to follow an optimal fuel consumption curve when generating required engine torque with resultant deterioration in fuel consumption.
To address such an issue, Patent Publication 3 (Japanese patent Application Publication No. 2005-337491) discloses a technology as described below. That is, during the shifting of the step-variable transmission, even if the resulting speed ratio is varied step-by-step, the electrically controlled continuously variable transmission is caused to vary a speed ratio so as to suppress such a stepwise variation in speed ratio. This allows the drive system to have an overall speed ratio, established based on the speed ratio of the electrically controlled continuously variable transmission and the speed ratio of the step-variable transmission, which is caused to continuously vary. Such an operation results in a capability of minimizing the stepwise variation of the engine rotation speed on the stage before and after the shifting of the step-variable transmission for thereby suppressing the occurrence of shifting shock.
Like a structure disclosed in Patent Publication 3 set forth above, under a circumstance where the speed ratio of the electrically controlled continuously variable transmission is caused to vary so as to suppress the stepwise variation in the speed ratio of the step-variable transmission to cause the overall speed ratio of the drive system to be continuously varied, an issue is encountered as described below. That is, although no engine rotation speed is caused to vary or the relevant variation is minimized, the step variable transmission and the electrically controlled continuously variable transmission have rotary elements whose rotation speeds vary with a resultant occurrence of inertia torque. Thus, the output torque of the drive system fluctuates by a value corresponding to inertia torque.
Patent Publication 3 proposes a method of addressing such an issue by subtracting torque from torque, generated by the electric motor forming the electrically controlled continuously variable transmission, by a torque component equivalent to inertia torque mentioned above under a situation where the overall speed ratio of the drive system is reduced, i.e., when effectuating an upshift.
However, Patent Publication 3 does not disclose a method of concretely deciding a value of torque equivalent to inertia torque mentioned above.
Further, Patent Publication 3 discloses only a case of effectuating the upshift with the overall speed ratio of the drive system being decreased and no attempt has been made to propose a method of addressing the same issue as that caused when a downshift is effectuated with the overall speed ratio of the drive system being increased.
Furthermore, under a circumstance where the downshift corresponds to a so-called coast downshift, i.e., a downshift in which a vehicle runs with an accelerator opening is zeroed or nearly zeroed, at least one electric motor is placed in a regenerative state in the hybrid vehicle drive system, causing an issue to easily arise with the occurrence of a drag of the engine.